MJE350 pinout and ccs issue - diyAudio
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Old 4th February 2006, 10:18 PM   #1
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Default MJE350 pinout and ccs issue

I am not sure where to put this. I built a bottlehead c4s ccs kit to use as a plate load for a preamplifier. The schematic of the C4S can be seen in this thread Some questions about C4S CCS adjustment. and my preamp uses a similar schematic, but uses a 6080 for the tube instead.

Almost all of the voltages check out and the led's all light up, but the voltage to the plate is 0. The ST datasheet shows one pinout (http://www.st.com/stonline/books/pdf/docs/4171.pdf) while the ON Semi shows a different one (http://rocky.digikey.com/WebLib/On-S...ata/MJE350.pdf). Mine is assembled according to the ST version as that's what the bottlehead instructions suggest.

So,does anyone have any experience to suggest that I should just turn the transistor around, or, if it is backwards would the high voltage hae killed it already, or could this be an error in the datasheet, or are there any other suggestions? There do not appear to be any markings on the transistors to indicate who the manufacturer is.

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Old 5th February 2006, 12:12 AM   #2
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How curious. I've only ever used the On Semi versions, so this does seem strange that a part with the same number has a different pinout. Did you get a kit of parts from Bottlehead, or just a pcb?
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Old 5th February 2006, 12:21 AM   #3
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I don't think the datasheets are in error, the ST is a SOT-32 package, and the ON Semi is TO-225. That's the reason for the different pinouts. Just sort out which part you have and plug it into the PCB accordingly. You should be able to twist it enough so the leads won't short.
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Old 5th February 2006, 01:18 AM   #4
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Originally posted by pinkmouse
Did you get a kit of parts from Bottlehead, or just a pcb?
I got the parts from bottlehead so I don't know who the manufacturer is. I have to admit, I basically understand how a triode works so I feel pretty comfortable working with them, but this solid state stuff is just magic. I tried turning one of the existing transistors around and still nothing. But then, I have no idea if the parts started bad, are now bad due to high voltage, of that I've done something else wrong. It is an incredibly simple little thing to assemble, so it is hard to imagine I did it wrong, but I am capable of screwing up even the simplest of tasks. For what it's worth, there has not been any smoke yet, so that's something. I guess the next step is to order a handful of transistors and keep trying.
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Old 5th February 2006, 04:38 AM   #5
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The MJE is a wierd part. ECB! Which is backwards from most power bipolars. And that is with the barely exposed metal side as back, reading left to right, pins down. Lettering front.

Chances are good that it is fine. In fact, it is still a transistor backwards, just with very low gain. Typically the beta is about 1. There are times and applicaitions where running a transistor backwards is a good idea (not this one). For example, the BC junction forms a very fast diode (leave E open). Or make a zener by avalanching the BE junction backwards.

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Old 5th February 2006, 05:27 AM   #6
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I have a cheezy little meter with a hFE setting on it. I connected the MJE350, and in one direction the hFE measured as 1, and in the other it was 0. I dug another PNP transistor out of my can-o-parts, and it measured 243 in one direction (seemingly showing that the meter works) and 1 in the other. This is promising as it seems likely to be bad transistors and not somethig wrong with the rest of the circuit. I'll order up some new ones and see what happens.
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Old 5th February 2006, 06:23 AM   #7
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TO-126 (TO-225, SOT-32, etc...) bipolar transistors are almost always ECB.

On the other hand, TO-220 bipolars are almost always BCE.
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Old 10th February 2006, 04:09 AM   #8
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Update but still not working. I posted this over at the Bottlehead site, but I have a sense I'll get a better response here:

I build a version of Pete Millett's low mu preamp. It uses a single power triode (6as7g) with a CCS as a plate load. Here is the schematic I am using:

Click the image to open in full size.

The B+ measures fine (about 160V). The diodes light up, and on the emitter of Q2 -- the lower transistor -- there there is about 158V. At terminal B (the collector of Q2, (this is where the output is taken from)) I measure about -0.9V, and from cathode to ground is about 0.1V. (as I look at that, is it possible the output coupling cap is toasted? I pulled it from a working preamp, but that doesn't necessarilly mean anything as I could have damaged it.)

I pulled out the first pair of mje350's and they measured dead in a hFE meter. I bought new ones, they measured fine, and put them in with the same results (actually, with the dead ones I got 0V, with the new ones I got -0.9V.) I next replaced the 2n2907a's, but no change.

The tubes light up (the 6080 has a seperate fillament transformer), the heaters have power, the led's light up, and I am absolutly stumped as to what is going on. Can anyone offer some some trouble shooting suggestions?


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Old 10th February 2006, 06:32 AM   #9
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Sigh, problem solved. The resistor that sets the current was off by a factor of 1000.
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Old 10th February 2006, 07:12 AM   #10
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